CLAYS OF KEW YOKK 813 



cooling of porcelain in a muffle kiln the iron is changed back to 

 the ferric condition with its accompanying yellow. 



When a kiln full of ware is finished, the material at times has 

 to be sorted, as it seldom happens that all the ware drawn from the 

 kiln is perfect. The sources of flaws in the burned ware may be 

 either faults in the body or bad firing. 



In connection with body faults: the more plastic and finer 

 grained the clay mixture used, the quicker it shrinks in drying; 

 masses which are fat shrink more than those which are rich in 

 fluxes, such as feldspar, or are very lean. The size of the quartz 

 and feldspar grains is of importance, for, if they are in the form of 

 fine powder, they are not very plastic, but if ground extremely fine 

 they develop a certain amount of pastiness, and this is accompanied 

 by an increased shrinkage. If the clay mixture was not properly 

 worked, or was too soft, or the thickness of the molded object is not 

 the same throughout, or the mechanically combined water is not 

 evenly distributed through the material, the ware is very likely to 

 warp in burning. The shrinkage may also be uneven if the 

 pressure exerted by the -molder is not uniform, and cracks occur 

 when the molded piece is stronger on one side than on the other. 

 Flaws, such as air bubbles, appear only when the ware is burned. 



Firing errors are usually due to too quick heating or cooling. 

 If cracks are caused in the early part of the burning, they increase 

 as the firing proceeds. Cracks formed in the body as a result of 

 too rapid cooling are not generally seen with the naked eye, but the 

 ware produces no ring when struck. Another cause of cracking is 

 an uneven temperature on the two sides of the object. Over burn- 

 ing as well as under burning of porcelain tends to produce fine 

 cracks in the body. 



The glaze is also a source of much worriment to the manufac- 

 turer. It should of course have the same coefficient of expansion as 

 the body to which it is applied. If under burned the glaze mil not 

 appear thoroughly glassy and develops fine cracks, but, if over 

 burned, a chemical action is apt to take place between the glaze 



